In the U.S., Latinos report higher rates of inactivity and related health conditions (e.g., hypertension, hypercholesteremia, obesity, heart disease, stroke, and diabetes) than non-Hispanic whites. Furthermore, Latino women (Latinas) report higher rates of sedentary behavior than Latino men. Due to cultural factors, socioeconomic circumstances, differences in educational background, and language barriers, Latinos in general, and especially Latinas, have limited access to public health interventions that promote physically active lifestyles. To address this public health crisis, effective interventions that leverage state-of-the-art theory and methods are needed. Our research group has 20 years experience developing and evaluating individually tailored computer expert system-driven physical activity interventions (based on Social Cognitive Theory and the Transtheoretical Model). In our recent pilot study (R21NR009864), we culturally and linguistically adapted our existing tailored intervention for sedentary Latinas and conducted a small randomized trial of the modified program (N=94). Six month (post-intervention) assessments are still ongoing, but three month data have been collected on 87 participants (93 percent retention). Intervention participants increased their physical activity from a mean of 17 minutes per week (SD=25.8) at baseline to 92 minutes (SD=69.8) at three months, whereas contact control participants reported increasing their physical activity from 12 minutes per week (SD=21.8) at baseline to 64 minutes (SD=84.3) at three months. The trends in the means at 3 months lend preliminary support for the efficacy of our intervention and the high retention rates, especially for a study targeting a hard-to-reach group like Latinas, bode well for its feasibility and acceptability. For the current proposal, we will build on these encouraging results by conducting an adequately powered (N=312) randomized controlled trial to test the efficacy of the culturally and linguistically modified, individually tailored print intervention compared with a wellness contact control condition (e.g., cardiovascular health information from NHLBI in Spanish). Data will be collected at baseline, 6, and 12 months using well-established physical activity measures (7-Day PAR, Actigraphs), as well as a comprehensive set of psychosocial questionnaires. We hypothesize that at the end of treatment (month six) intervention participants will report significantly more minutes of at least moderate intensity physical activity per week than the wellness contact control participants. We will also examine the maintenance of treatment effects at 12 months, costs of delivering the tailored intervention program, and potential mediators and moderators of the intervention-physical activity relationship. In the proposed study we seek to promote physical activity among this at-risk population using a high-reach, low-cost strategy, which has great potential for adoption on a larger scale and thus high potential for reducing existing health disparities in the U.S. PUBLIC HEALTH RELEVANCE: As Latinos are the fastest growing minority group in the U.S. and report high rates of inactivity and related health conditions, we will conduct a large scale randomized controlled trial to test the efficacy of a culturally and linguistically modified, individually tailored physical activity intervention among Latinas (n=312). The proposed study seeks to promote physical activity among this at-risk population using a high-reach, low-cost strategy, which has great potential for adoption on a larger scale and thus high potential for reducing existing health disparities in the U.S.